Field of the Invention
The invention relates to a method for operating a resonance-measuring system, in particular a Coriolis mass flowmeter, wherein the resonance-measuring system includes at least one controller, at least one electric setting device, at least one oscillation element and at least one oscillation sensor, wherein the controller generates a controller output signal u1 for controlling the electric setting device, the electric setting device provides an electric excitation signal for exciting the electromagnetic drive, the electromagnetic drive excites the oscillation element into oscillation in at least one natural mode and wherein the excited oscillation of the oscillation element is detected by the oscillation sensor and is output as at least one output signal. Furthermore, the invention also relates to a resonance-measuring system with which the above-mentioned method can be carried out.
Description of Related Art
Resonance-measuring systems of the above-mentioned type have been known for many years, not only in the form of Coriolis mass flowmeters, but also as density-measuring devices or fill level monitors using the tuning-fork principle, as quartz scales and band viscometers, etc. These resonance-measuring systems are related to a process, wherein process and resonance-measuring system interact.
In the following, resonance-measuring systems are described using the example of Coriolis mass flowmeters, which is to be understood as not be limiting of the invention. In the present, resonance-measuring systems are identified in general as such systems in which information concerning the process variables to be determined (measuring variables) are encoded in the natural modes and/or such systems in which the working point is set at the natural mode of the measuring system. The following designs are applicable to all systems falling under this definition. In Coriolis mass flowmeters, the measuring tube corresponds to the oscillation element of the resonance-measuring system; this particular design of the oscillation element also does not present a limitation for the general teaching being applicable for resonance-measuring systems.
Resonance-measuring systems in the form of Coriolis mass flowmeters are used in industrial process-measuring technology primarily where mass flow needs to be measured with a high accuracy. The functionality of Coriolis mass flowmeters is based on at least one measuring tube—the oscillating element—with medium flowing through it being excited to oscillation by an oscillation generator, wherein this oscillation generator is designed on the premise of being an electromagnetic drive. In such an electromagnetic drive, a coil normally has electric current flowing through it, wherein a force acting on the oscillation element is directly associated with the coil current. In Coriolis mass flowmeters, the functionality is based on the massive medium retroactively affecting the walls of the measuring tube due to the Coriolis force caused by two orthogonal movements—that of the flow and that of the measuring tube. This retroaction of the medium on the measuring tube leads to a change in the measuring tube oscillation compared to the oscillation state of the measuring tube without flow. By detecting these characteristics of the oscillation of the Coriolis measuring tube with flow, the mass flow through the measuring tube can be determined with high accuracy.
The natural modes of the Coriolis mass flowmeter or the oscillateable parts of the Coriolis mass flowmeter are of particular importance, essentially the natural modes of the measuring tube as oscillation element, because the working point of the Coriolis mass flowmeter is normally set at the natural mode of the measuring tube in order to be able to impress the necessary oscillations for the induction of the Coriolis force with a minimum energy input. The oscillations then carried out by the measuring tube have a certain form, which is called the natural mode of the respective excitation.
It is known from the prior art for the controller to generate a harmonic base signal as controller output signal in the form of a sinusoidal voltage for exciting the oscillation element and this sinusoidal voltage activates the electric setting device, wherein the electric setting device has the task of providing a corresponding power at its outlet in order to be able to activate the electromagnetic drive in a suitable manner and with sufficient power. Thus, the electric setting device is, in practice, the performance link between the controller and the electromagnetic drive of the resonance-measuring system.
The controller serves the purpose of operating the oscillation element in resonance, whereto it must be determined whether input and output values of the resonance system have a phase difference corresponding to the resonance. In the case of Coriolis mass flowmeters, on the input side, this is the force with which the measuring tube is excited as oscillation element and, on the output side, this is the velocity of the measuring tube. Due to the correlations forming the basis of this oscillateable system, a resonance is present when the input-side force and the output-side measuring tube velocity have a phase difference Δφ of 0°. If this phase requirement is fulfilled, the desired resonance is present. For this reason, the control loop for operating a resonance-measuring system of this sort known from the prior art is a phase-locked loop.
Due to the correlation between the flow through a drive coil of the electromagnetic drive and the effective force as electric setting device or as part of an electric setting device, resonance-measuring systems having an electromagnetic drive often have a voltage-controlled current source, which must have a large bandwidth and should barely cause any additional phase shift in the frequency working range. The phase control is thus normally based on a phase measurement between the measuring tube velocity and the driving voltage of the electric setting device with the assumption that the influence of the setting device and/or the electromagnetic drive itself is negligible on the phase difference. This is problematic in various ways.
The impression of the current in the electromagnetic drive having a coil inevitably leads to voltages at the drive coil that are too high and noisy, since the jumps in the controller output signal—even if these are only caused by the quantization stages of a digital/analog converter—occur as jumps in current through the electromagnetic drive and are “differentiated” there by the drive coil; this holds true, in particular, for setting devices with a high slew rate, i.e., with a high increasing velocity of the current. This is problematic in view of the electromagnetic compatibility and also leads to a decrease of the signal to noise ratio and thus to an increase of the measuring inaccuracy in measuring different process variables—mass flow in the case of Coriolis mass flowmeters—and in determining different parameters of the resonance-measuring system—for example the stiffness of the measuring tube in the case of Coriolis mass flowmeters. For this reason, a quick measurement of the drive current is also not possible, since the relatively small drive current is very noisy, which requires a long averaging time until a sufficiently smooth signal is obtained.